This invention relates to a method of optimizing performance characteristics of internal combustion engines and more particularly to a method of optimizing performance and fuel economy with emissions constraints in a lean capable engine that can operate in multiple combustion modes having a time variant after-treatment system.
Manufacturers have been continuously improving the performance of internal combustion engines. In order to meet ever-increasing standards for fuel economy and vehicle emissions, however, manufacturers have been forced to consider new methods for increasing fuel economy and reducing undesirable fuel emissions. One improvement being considered is a lean capable engine, such as a direct injection engine, that can operate in multiple combustion modes.
Conventional internal combustion engines use fuel injectors to precisely control the amount of fuel inducted into the engine""s cylinders. Also, fuel injectors atomize the liquid fuel, increasing the homogeneity of the air and fuel mixture. In conventional internal combustion engines, this air and fuel is mixed prior to entering the combustion chamber.
In contrast, in a direct injection engine, fuel and air mix in the combustion chamber itself. The primary benefit of this is that the fuel burns more thoroughly, and correspondingly delivers more power and fuel economy as compared to a conventional internal combustion engine.
Lean capable, multiple combustion mode engines, such as a direct injection engine, can provide power in three basic combustion modes, those being homogeneous stoichiometric, homogeneous lean, and stratified.
The homogeneous stoichiometric mode can be used under almost any operating condition. During homogeneous stoichiometric operation, the engine operates at an air/fuel ratio (AFR) near stoichiometry or approximately 14.6:1.
The homogeneous lean mode, on the other hand, is desirable only at moderate engine loads. During homogeneous lean operation, the engine operates at an AFR of approximately 18:1 to 25:1. As the engine load increases, however, the homogeneous lean mode is limited by the engine""s ability to produce torque. In addition, at the lower end of engine loads, the homogeneous lean mode is limited by combustion stability.
The stratified mode is desirable only at lower engine speeds and torque operating points. High load operation may result in undesirable hydrocarbon (HC) and smoke emissions. Unlike the homogeneous lean mode, however, the stratified mode can be used at very low engine loads, including idle. Stratified operation is characterized by an overall AFR between approximately 25:1 and 40:1.
These various combustion modes have an effect on the exhaust gas emissions of lean capable, multiple combustion mode engines. Typically, an additional three-way catalyst is positioned downstream of a first three-way catalyst. The additional catalyst, sometimes referred to as a lean NOx, trap (LNT) is periodically purged by operating the engine at a rich air/fuel ratio to release and reduce stored NOx. This is referred to as a time-variant after-treatment system because the LNT efficiency, and hence, the efficiency of the after-treatment system, changes with time as the LNT fills with NOx.
Thus, there exists a need for optimized mode scheduling to minimize fuel consumption and minimize exhaust gas emissions for lean capable, multiple combustion mode engines having a time-variant after-treatment system.
It is an object of the present invention to provide an optimization methodology to determine the optimum transmission gear, combustion mode, air/fuel ratio (AFR), spark advance, and amount of exhaust gas recirculation (EGR) for emissions-constrained lean capable, multiple combustion mode engines having a time-variant after-treatment system.
The above and other objects and advantages are achieved by providing a method of determining a desired transmission gear, combustion mode, AFR, spark advance, and EGR rate for all of the possible engine speed and wheel torque values. The method comprises the steps of, starting at the vehicle level, determining the range of speeds and wheel torques. A cost value, which is a function of fuel economy and emissions, is then initialized at a large value for spark advance, EGR, AFR, combustion mode, and transmission gear (respectively Jspark, Jegr, Jafr, Jmode, and Jgear). For each combustion mode (homogeneous stoichiometric, homogeneous lean, or stratified) at each transmission gear, the AFR, EGR and spark advance are varied to minimize the cost values associated with fuel flow and emissions with respect to target values. A cost value for the evaluation (Jeval) is calculated using a Lagrangian weighting factor. The resulting values are considered a local minimum. A repetitive process of comparing the various cost values to one another (i.e. Jeval to Jspark, Jspark to Jegr), adjusting cost values if necessary and recalculating Jeval, is begun and continues until such time as a global solution is achieved. The global solution is defined as the optimum fuel economy and emissions in terms of cost value (Jfinal) for each vehicle speed and wheel torque at each point of an operating parameter grid (transmission gear, spark advance, AFR, EGR, and combustion mode).
This process accounts for the time-varying after-treatment system by determining the amount of time the engine can spend in lean or stratified operation, and a LNT purge time by calculating a weighted average of the emissions from the lean or stratified mode and purge operation. By assuming a steady-state operation at each speed-load point, and by applying the same Lagrangian weighting factor for all vehicle operation modes, the optimization method, which is not cycle specific, provides calibrations with no defeat device like characteristics.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.